This invention relates to a melt spinning process for production of fully oriented crystalline synthetic filaments with high mechanical properties. More specifically, the present invention provides an improved process for melt spinning fiber-forming synthetic polymers which produces filaments with a very high degree of orientation, high crystallinity, low shrinkage, and high tenacity.
The typical melt spinning processes used commercially in the production of filaments or fibers from fiber-forming synthetic polymers may be characterized as two-step processes. The molten polymer is extruded through spinneret holes to form filaments, and then in a separate step, performed either in-line coupled with the extrusion step or in a separate subsequent operation, the filaments are stretched or drawn to increase the orientation and impart the desired physical properties. For example, commercial polyester filaments, such as polyethylene terephthalate (PET), have for many years been produced by a two step process in which the polymer melt is extruded through a spinneret to form filaments and after solidification, the filaments are wound up at speeds on the order of 1000 to 1500 m/min. The as-spun fibers are then subjected to drawing and annealing at speeds on the order of 400 to 1000 m/min. The handling, energy and capital equipment requirements for such two-step processes contribute significantly to the overall production cost.
In order to reduce production cost and increase production rate, it would be desirable to develop a process for producing fully oriented crystalline PET fibers in a single step with properties equivalent to or better than those produced by the conventional two-step processes. To this end, a number of researchers have explored technology based on high speed spinning. In 1979, DuPont [R. E. Frankfort and B. H. Knox, U.S. Pat. No. 4,134,882] documented a process based on high speed spinning technology at speeds up to about 7000 m/min, providing oriented crystalline PET filaments in one step having good thermal stability and good dyeing properties. However, the fibers have mechanical properties still inferior to those of fully drawn yarns produced by the conventional two-step process.
Parallel to the above study, reports on high speed spinning research can be found elsewhere in the literature since the late 1970's. Properties and structure of high speed spun PET fibers are well characterized. Typical characteristics of high speed spun fibers are lower tenacity, lower Young's modulus and greater elongation as compared with conventional fully oriented yarns [T. Kawaguchi, in "High Speed Fiber Spinning", A. Ziabicki and H. Kawai, Eds John Wiley & Sons, New York, 1985, p. 8]. More recently, a take-up speed up to 12,000 m/min for spinning PET has been reported. But, heretofore it has not been possible to produce as-spun PET fibers by superhigh speed spinning that have properties equivalent to those of conventional two-step spun fibers. Moreover, the orientation and crystallinity of as-spun fibers, respectively, reach maximum values at certain critical speeds, above which severe structural defects such as high radial non-uniformity and microvoids start to develop, which materially restrict attainment of high performance fibers.
Our objective in the present invention is similar to that of the above-noted researchers: namely, providing a process for producing fully oriented crystalline fibers in a single step with properties equivalent to or better than those produced by the conventional two-step processes. However, in pursuing this objective, we have departed from the path followed by the above-noted researchers. Instead of continuing the investigation of high speed spinning, this invention modifies the threadline dynamics of the spinning operation to produce high performance fibers in a one-step process.
It was revealed in our previous work [Cuculo, et al. U.S. Pat. No. 4,909,976, granted Mar. 20, 1990] that fiber structure (orientation and crystallization) development along the fiber spinning threadline can be significantly enhanced by optimizing the threadline temperature profile. This was achieved by introducing a zone cooling and zone heating technique to alter the temperature profile of the spinning threadline to enhance the structure formation. Take-up stress remained almost unchanged as compared with that of conventional spinning.